Light olefins such as ethylene and propylene have traditionally been manufactured by cracking various hydrocarbon streams, ranging from gases such as ethane, to liquid fractions, including relatively low boiling point liquids such as naphtha to relatively high boiling point liquids such as gas oils. Gas oils typically have a final boiling point of up to 340° C. (650° F.), being derived typically from an atmospheric pipestill sidestream located just above the bottoms product. The atmospheric still bottoms product is commonly termed as “atmospheric resid” or “long resid.” Atmospheric resid can be provided to a vacuum pipestill operating at lower hydrocarbon partial pressures, albeit at an additional economic cost to do so. The non-bottoms products of a vacuum pipestill may be referred to as “vacuum gas oils” and typically have a final boiling point of up to 650° C. (1050° F.). The bottoms product of a vacuum still is known commonly as “vacuum resid,” “short resid,” or “pitch.”
Steam cracking (“cracking”) generally entails heating hydrocarbon streams in the presence of steam (or other generally inert substance such as methane), in a steam cracking furnace, typically to a temperature in excess of about 370° C. (700° F.) and 25 psia. At such conditions, many of the hydrocarbon molecules undergo cracking, that is, the breaking of carbon-carbon bonds and/or releasing hydrogen from saturates to form ethylene and propylene, among other olefinic and aromatic products. Through undesirable side reactions, the furnace tubes will gradually accumulate carbonaceous deposits or “coke.” Coke build-up eventually causes an unacceptable increase in furnace pressure drop and loss of heat transfer, and periodically the furnace must be taken out of service to undergo a steam-air decoking operation to remove the coke deposits from the inside of the tubes. Generally, the higher the final boiling point of the feedstock, the higher the content of species that increase the rate of coking in the furnace tubes, particularly asphaltenes or multi-ring aromatic species. Feedstocks having components with a final boiling point above 500° C. (932° F.) and even more so above 565° C. (1050° F.) can cause furnace run-lengths to drop to a week or less and are thus generally unacceptable as a feedstock. However, it is otherwise desirable to use such heavy feedstocks as cracker feed because they still contain a significant proportion of crackable components. Further, such feedstocks are typically inexpensive relative to lower boiling range counterparts (e.g., naphtha) and are readily available in some regions of the world. The challenge is to maximize the amount of crackable components while retaining an overall economic cost advantage. These motivations are also applicable to heavy feedstocks that have undergone minimal processing, such as non-processed whole crudes and atmospheric resids that avoided the expensive vacuum pipestill step and still contain substantial amounts of crackable molecules.
Thus, production of olefins from steam cracking of heavy hydrocarbon feedstocks remains an area of increasing industrial importance and methods have been disclosed for such. One such method involves introducing a flash operation within the convection sections of a pyrolysis furnace, where a heavy feedstock and steam mixture is preheated and separated. The overhead flash vapor of the heated mixture is then further heated to a higher temperature in the radiant section such that cracking occurs and olefins are produced. The flash operation produces a bottoms liquid product containing most of the problematic higher boiling point components that are not cracked. Such method is generally more efficient in providing useful, crackable molecules to the radiant section of the furnace than a typical vacuum pipestill operation, by virtue of the much higher steam content of the mixture in a pyrolysis furnace and the attendant lower hydrocarbon partial pressure in the flash operation. Useful methods and apparatus for conducting such flash operation are found for example in U.S. Pat. Nos. 6,632,351, 7,097,758, and 7,138,047. However, the liquid bottoms of such operations are typically very heavy and of particularly low value, suffering undesirable characteristics such as high viscosity and concentrated, high levels of sulfur, nitrogen, metals, or other undesirable inorganics.
Various methods have been contemplated to address this low-value bottoms product aspect, instructing one or more operations on the feedstock to make a higher percentage suitable for steam cracking. For example, U.S. Pat. No. 4,065,379 suggests thermally cracking an atmospheric resid stream at moderate temperatures, separating a gas oil stream from the product of the thermal cracking, catalytically hydrotreating the gas oil stream, and then steam cracking the hydrotreated gas oil stream. The beginning thermal cracking step generates a high viscosity secondary residue that is of lower quality than vacuum pipestill bottoms, which is disposed of as a fuel. Presently, such disposition is environmentally unacceptable and significant additional treatment or dilution with low sulfur, low viscosity materials would be required for use as a fuel.
As the art progressed, the issue of secondary bottoms residue was further addressed, but again directed some form of rather complex treatment of the feedstock prior to steam cracking. U.S. Pat. No. 4,309,271 refers to hydrogenation of a high boiling feedstock, optionally with fractionation to remove remaining high boiling components, and providing the distilled, hydrogenated materials to a steam cracker. U.S. Pat. No. 6,303,842 suggests hydrotreating and/or solvent deasphalting a heavy feedstock prior to stream cracking the appropriate fractions derived from the hydrotreating or deasphalting process. Note that solvent deasphalting also generates a high viscosity, high sulfur, asphaltene laden, environmentally challenged secondary residue that is difficult to use as a fuel. Similarly, U.S. Patent Applications 20070090018, 20070090019 and 20070090020 direct one to hydroprocess a heavy feedstock and provide the hydrogenated feedstock to a steam cracking furnace comprising a flash operation such as found in U.S. Pat. Nos. 6,632,351, 7,097,758, or 7,138,047, noted above. A secondary residue is generated as the flash liquid bottoms product that is of less environmentally challenged quality with a higher fuel value than the heavy feedstock that had not first been hydroprocessed. However, a significant problem with each of the above references is that they conduct one or more rather complex, costly treatments to the entire feed stream, which feed includes a great quantity of already useful, crackable material that derives little or no steam cracking benefit from such treatment and unnecessarily consume treatment feeds and resources such as hydrogen, steam, and heat.
In another attempt to overcome the above problems, U.S. Pat. No. 3,617,493 suggests reheating and flashing the liquid bottoms stream from a first flash separator in a second flash separator but otherwise does little to improve the ability of the second flash separator to improve the crackable fraction of the feed stream. Such arrangement produces a highly undesirable bottoms product from the second flash separation and does very little to upgrade the overall crackable quality of the feedstock or to prevent formation of asphaltenes, tars, and coke precursors. Still another problem confronting olefins producers is the increasing cost or in some instances simple unavailability of suitable fuel streams, particularly gaseous fuels streams required to power and operate a pyrolysis furnace, ancillary boiler furnaces, and other equipment.